1. Field of the Invention
The present invention relates generally to friction stir welding tools. More particularly, this invention relates to an improved pin profile for a friction stir weld tool tip.
2. Description of Related Art
Friction stir welding ("FSW") is a welding technique that has only recently been developed. This technique was developed primarily for welding metals and alloys that heretofore had been difficult to weld using more traditional fusion techniques. Aluminum and aluminum alloys, in particular, are difficult to weld, requiring weld pool shielding gas and specialized heat sources, and require the oxide layer to be stripped prior to or during the welding process. In addition, aluminum and its alloys are subject to voids and solidification cracking defects when they cool from a liquid. Consequently, in order to manufacture large panels of aluminum and aluminum alloys, extrusion has become the choice of manufacturing. However, even extrusion techniques have size limitations. FSW, as a mechanical stirring technique, is a solid phase welding technique that is a simple and efficient method for obtaining satisfactory welds when using aluminum and its alloys.
Friction stir welding technique uses a rotating shouldered cylindrical tool with a projecting distal pin to create mechanical friction in the metal in contact with the rapidly rotating cylindrical pin tool. This mechanical friction plasticizes the metal. For FSW, the pieces to be welded are clamped to a backing plate with the faying surfaces in close abutment. The pin of the rapidly spinning cylindrical tool is brought into contact with the metal and, for butt joint welding, is centered with the center line of the joint. This contact rapidly creates friction heating, plasticizing the metal, and the pin is slowly plunged into the joint line until the shoulder of cylindrical tool contacts the surface of the metal. At this stage there is a substantial amount of plasticized metal in a column about the rotating pin beneath the shoulder of the cylinder. Parameters for the technique are chosen, such as pin rotation speed, size of the shoulder in relation to the pin diameter, plunging force and rate of translation of the pin, so that a sufficient amount of metal is stirred and the temperature of the metal stays below the metal's melting temperature.
The pin is then moved relative to the work piece along the line of the joint. As the rotating pin moves, the plasticized metal is extruded to the back face of the pin while undergoing a mechanical stirring and forging action imparted by the pin surface profile and confined from above by the pressure exerted on the material by the shoulder of the cylindrical tool. Metal encountering the leading face of the pin is crushed, heated and plasticized only to be extruded to the back face as the pin proceeds down the joint line. Thus, FSW crushes metal along the joint line, breaking up the oxide film, and stirs the plasticized metal on the trailing side of the pin under the shoulder of the cylindrical tool where the metal begins to cool forming a weld. Because the metal is heated to a point below the melting point, this is a solid-phase weld.
Friction stir welding avoids several drawbacks of fusion welding, such as gas metal arc welding. Fusion welding requires that the metal be liquefied forming a weld pool which then cools to a weld bead running the risk of creating voids and cracks. In addition, metal fumes are given off, and in the case of alloys, the metal composition of the weld most likely changes compared to the native alloy because of differential evaporative losses of the alloy constituents. Fusion welding techniques may also lead to segregation of the alloy constituents. In the case of difficult metals such as aluminum, a protective weld gas shield is also required. Friction stir weld does not require the addition of filler or other consumables as some fusion welding techniques do. Additionally, FSW also exhibits superior weld strength and fatigue life compared to gas metal arc welding.
U.S. Pat. No. 5,460,317 titled FRICTION WELDING and issued to Thomas, et al., on Oct. 24, 1995, discloses a method of friction stir welding using a spinning cylindrical tool with pin or an oscillating flattened blade for welding. The pins described are generally smooth except for one embodiment disclosing a pin having a complex surface topography for forming a local plasticized zone in a single locality with the purpose of leaving the plug in place to complete the "spot" weld.
In general, the performance of friction stir welding has exceeded the performance traditionally attributed to more conventional fusion welding techniques. In a paper by Midling, Ole T., and Johansen, Helge G., entitled "Production of Wide Aluminum Profiles by Solid State Friction Stir Welding" as a presentation at the Sixth International Aluminum Technology Seminar and Exposition, pp. 1-10 (Chicago, Ill., May 1996), the authors discuss friction stir welding in comparison with more traditional welding methods. The mechanical properties of the welds are compared for a number of different aluminum alloys demonstrating a superior performance in weld strength for both bending and tension. The authors also note that the profile of the pin surface is important for controlling the degree of mixing and ultimately the strength of the weld. The authors describe a pin profile having two circumferential fins projecting from the center of the pin.
In an article titled "Friction Stir Process Welds Aluminum Alloys" by C. J. Dawes and W. M. Thomas, Welding Journal, vol. 75, no. 3, pp. 41-45 (March 1996), the authors discuss friction stir welding trials that had been conducted on various alloys of aluminum, including aluminum and copper, aluminum and magnesium, and aluminum, magnesium and silicon alloys. They were able to demonstrate that friction stir welding can weld metals that otherwise cannot be welded with fusion welding techniques. These welds have high joint strengths and do not suffer from porosity. In addition solid-phase welding enables the retention of the metallurgical properties of the alloys because there is no evaporation of constituent components as would occur in fusion welding. In addition, the authors described the added advantages of welding dissimilar materials and the ability to weld many different component shapes that would normally not be practical or cost effective to either extrude, cast or use fusion welding techniques to manufacture.
Friction stir welding is suitable for use with a number of joint configurations including: square butt, combined butt and lap, single lap, multiple lap, three piece T-butt, two piece T-butt, and edge butt. Of these joints, the square butt, the butt portion of the combined butt and lap, and the three piece T-butt have joint surfaces that are parallel to the axis of the stirring pin. The mixing action is from side to side and top to bottom and will remain fairly uniform and symmetric for as long as the parameters for the welding are kept constant and, for butt joints, if the pin stays on the center line of the joint.
For lap joints, the faying surfaces of the joint are transverse to the axis of the stirring pin and the mixing must be accomplished in the up and down directions. Current FSW practice uses a pin having a surface profile consistent with the thread of a bolt, much like the end of a machine bolt. These pins experience difficulty when attempting to weld a lap joint. The difficulty encountered is interface deformation which is diagrammatically depicted in FIG. 1 by the letter "D". Note that the deformation also is not symmetrical from side to side. With interface deformation, the faying surfaces of the pieces to be welded have been deformed in one direction without fusion occurring between the pieces in this area. This area is also known in the art as a thinning joint. This deformation effectively results in thinning of the affected piece, noted by the letter "T" in the diagram. The thinning substantially weakens the piece, particularly when a pealing shear force is applied to the work piece. Reversing the direction of spin of the stirring pin does not correct this. Instead, the deformation occurs in the opposite direction.
Accordingly, it is an object of the present invention to provide a shouldered cylindrical stirring pin tool that can accomplish adequate friction stir welding for all joint configurations.
It is a further object of the present invention to provide a complex stirring pin profile that is suitable for obtaining adequate friction stir welding for all joint configurations.